1. Field of the Invention
The present invention relates to generation processing of a halftone screen and image processing using the halftone screen.
2. Description of the Related Art
An electrophotographic printing apparatus, which has an exposure process for removing charges from a uniformly charged surface layer of a photosensitive drum, which is made up of an organic photoconductor (OPC), amorphous silicon, or the like, by scanning a light beam, has a non-linear characteristic. Also, complexity of an electrophotography process including development, transfer, fixing, and the like also causes the non-linear characteristic.
With this non-linear characteristic, an interference occurs between print dots. For example, when one isolated dot is to be printed, it is difficult to record such a dot, but dots are reliably recorded in a cluster state of several dots. When a distance between dots is small, toner often moves to connect dots. In a process for recording dots by attaching ink droplets on a medium like in an ink-jet system, although a micro phenomenon between the ink and medium occurs, an interference between dots hardly occurs and dots can be surely recorded.
A print image by an electrophotographic printing apparatus is largely influenced by variations of a spatial frequency due to the non-linear characteristic of the electrophotography process. Upon forming a halftone image by the electrophotography process, a halftone dot method is used in consideration of the non-linear characteristic. When the halftone dot method is used, a fundamental spatial frequency is fixed, and dots can be stably recorded without being influenced by variations of the spatial frequency. For example, assuming that the screen ruling of halftone dots is N lines/inch, a pitch P of halftone dots is 25.4/N mm. That is, the spatial frequency is 1/(2P)=N/(2×25.4), and the fundamental spatial frequency is fixed. Therefore, when a printing apparatus is designed to always stabilize the electrophotography process at the fundamental spatial frequency, a print image can be stably formed. For example, in a printing apparatus of 1200 dpi, the fundamental frequency of a halftone dot screen of 200 lines/inch is 4 cycles/mm. That is, by stabilizing the electrophotography process at the spatial frequency of 4 cycles/mm, the image reproduction characteristic of the printing apparatus can be improved.
An AM modulation method based on the halftone dot method can obtain a stable image reproduction characteristic. At the same time, moiré is readily generated since C, M, Y, and K color tones are superposed in color printing. In order to suppress moiré, screen angles are changed for respective color components to drive moiré beats generated among color components toward the high frequency side, thus visually obscuring moiré. For example, moiré due to superposition of color toners is suppressed by setting the Y screen angle to be 30°, and the C, M, and K screen angles to be 0° or 60°.
In digital halftone processing, since the resolution of a digital image is discrete, arbitrary screen angles cannot be set. However, if optimal and discrete screen angles are selected for respective color components, moiré can be suppressed.
However, even when the technique that changes the screen angles is introduced, moiré beats are merely driven to the high frequency side, and a unique pattern formed due to superposition of color components remains. This is a so-called Rosetta pattern, which disturbs upon outputting a high-image quality image. Particularly, upon outputting a photo image with high image quality, smooth image quality reproduction is demanded like a photo obtained by the silver halide process, and the Rosetta pattern becomes a serious disturbance.
As another approach, a method of attaining tone reproduction by an FM modulation method based on error diffusion or blue noise masking is available. The FM modulation method is popularly adopted in an ink-jet system, thermal transfer system, and the like, since it generates a random layout of print dots, has high tonality, and is free from any moiré due to superposition of color components. However, in the FM modulation method, the dot interval changes, and cannot be freely controlled. For example, as the density value becomes higher, the dot interval is gradually reduced. For this reason, the spatial frequency characteristic changes to the high frequency side, and is directly influenced by the frequency characteristic of a printing apparatus. Therefore, the FM modulation method is not suited to an electrophotographic printing apparatus, which is readily influenced by spatial frequency variations.
As a method of solving the aforementioned problems, a hybrid halftone method attracts attention. This method is an intermediate method of the AM and FM modulation methods, and has features of both the methods. The hybrid halftone method clusters dots to vary dot intervals. Since the dot interval variations are irregular and anisotropic, the spatial correlation among dots lowers, thus suppressing generation of moiré.
The hybrid halftone method deprives periodicity by clustering dots and moving lattice points (the central positions of halftone dots) using a random number and rotation manipulation, thereby suppressing generation of moiré. Various methods of generating a hybrid halftone screen have been proposed. These methods have an advantage (print stability) of the AM modulation method and those (moiré dissolution, high resolution) of the FM modulation method, and an image output with high screen ruling and high image quality is expected.
However, the hybrid halftone method targets a printer of a high resolution (e.g., 2400 dpi), and is premised on formation of one halftone dot at a high resolution. On the other hand, an office-use electrophotographic printer has a resolution as low as 600 dpi, and it is difficult to achieve both high halftone dot screen ruling and a sufficient tone characteristic. Furthermore, heterogeneity in pitch of an engine that scans a light beam, banding noise due to mechanical vibrations, and the like cause considerable deterioration of image quality. Hence, in order to apply the hybrid halftone method to an office-use printer, special attention is needed.
Upon applying the hybrid halftone method to an electrophotographic printing apparatus, low and middle density ranges exhibit relatively good tone characteristics. However, in a high density range, halftone dots grow to reduce the areas of blank regions among halftone dots, and blank regions each having a width of one to two pixels are formed. For this reason, the blank regions become crushable and unstable owing to the temperature, mechanical vibrations, electrophotography process variations, and the like, and image quality considerably deteriorates due to the aforementioned heterogeneity in pitch and banding noise.
As a method of avoiding the blank regions from crushing, a method of introducing white dots is known. In the AM modulation method, black and white dots are laid out in a checkerboard pattern to blacken white dots from outside (to grow black dots) to have an intermediate density as a boundary. That is, black and white dots grow to be symmetrical about a density axis, so as to stabilize white dots. However, since white dots are laid out in a checkerboard pattern, the screen ruling of halftone dots is reduced to about √(½)≈0.7 compared to a case without introducing any white dots (all black dots).
When a printer of 600 dpi is used to form a halftone dot screen of 175 lines on average, about 12 pixels are used for a halftone dot that forms one threshold matrix using the threshold matrix of 3.4×3.4 pixels on average in a state without introducing any white dots. However, when white dots are introduced, a threshold matrix is defined by about 2.4×2.4 pixels, and only about six pixels are used for a halftone dot that forms one threshold matrix. That is, it becomes difficult to form a desired halftone dot screen, that is, it is difficult to introduce white dots to an office-use printer with a low resolution. In other words, stable tone reproduction with high screen running and high image quality is demanded for a low-resolution printer without introducing any white dots.